Commons

Lowering a weight example 9⊲ After lifting the weight, you need to lower it again. What’s happeningin terms of energy?⊲ Your body isn’t capable of accepting the energy and putting itback into storage. The gravitational energy all goes into heat.(There’s nothing fundamental in the laws of physics that forbidsthis. Electric cars can do it — when you stop at a stop sign, thecar’s kinetic energy is absorbed back into the battery, through agenerator.)v / Example 10.w / Example 10.Absorption and emission of light example 10Light has energy. Light can be absorbed by matter and transformedinto heat, but the reverse is also possible: an object canglow, transforming some of its heat energy into light. Very hot objects,like a candle flame or a welding torch, will glow in the visiblepart of the spectrum, as in figure v.Objects at lower temperatures will also emit light, but in the infraredpart of the spectrum, i.e., the part of the rainbow lyingbeyond the red end, which humans can’t see. The photos infigure w were taken using a camera that is sensitive to infraredlight. The cyclist locked his rear brakes suddenly, and skiddedto a stop. The kinetic energy of the bike and his body are rapidlytransformed into heat by the friction between the tire and the floor.In the first panel, you can see the glow of the heated strip on thefloor, and in the second panel, the heated part of the tire.Heavy objects don’t fall faster example 11Stand up now, take off your shoe, and drop it alongside a muchless massive object such as a coin or the cap from your pen.Did that surprise you? You found that they both hit the groundat the same time. Aristotle wrote that heavier objects fall fasterthan lighter ones. He was wrong, but Europeans believed him forthousands of years, partly because experiments weren’t an acceptedway of learning the truth, and partly because the CatholicChurch gave him its posthumous seal of approval as its officialphilosopher.Heavy objects and light objects have to fall the same way, becauseconservation laws are additive — we find the total energyof an object by adding up the energies of all its atoms. If a singleatom falls through a height of one meter, it loses a certain amountof gravitational energy and gains a corresponding amount of kineticenergy. Kinetic energy relates to speed, so that determineshow fast it’s moving at the end of its one-meter drop. (The samereasoning could be applied to any point along the way betweenzero meters and one.)Now what if we stick two atoms together? The pair has doublethe mass, so the amount of gravitational energy transformed into26 Chapter 1 Conservation of Mass and Energy